1. Field of the Invention
The present invention relates to a sheet conveyance apparatus configured to convey a sheet while pinching the same, and to an image forming apparatus, such as a copying machine, a facsimile apparatus, or a printer, equipped with the sheet conveyance apparatus.
2. Description of the Related Art
In an image forming apparatus such as a copying machine, a facsimile apparatus, or a printer, it can happen that the printing position of a printed image is deviated with respect to the sheet. This is mainly due to the fact that within the feeding cassette, for example, the sheet is deviated with respect to the image in the width direction, which is orthogonal to the conveyance direction, or that the sheet is arranged obliquely. Further, also in the conveyance path to be passed after the feeding, positional deviation in the sheet width direction or skew feed may be generated. In view of this, as configurations for achieving an improvement in terms of image printing accuracy, a skew feed correction through abutment of a sheet leading edge portion and a positional deviation correction through movement in the width direction of a registration roller, have been discussed.
Japanese Patent No. 4016621 discusses a registration configuration equipped with a skew feed correction mechanism through abutment of the sheet leading edge portion, in which the sheet leading edge portion is caused to abut on the registration roller to thereby effect skew feed correction. Specifically, the advancement side in the width direction of the sheet leading edge portion conveyed from a roller (hereinafter referred to as the upstream roller) provided on the upstream side of the registration roller first abuts on a nip edge line of the registration roller at rest. And, further, through excessive pushing by the upstream roller, a sheet loop is formed between the upstream roller and the registration roller. As a result, the sheet is turned within the loop, whereby transition of the abutment in the width direction of the sheet leading edge portion to the delay side is gradually effected. Eventually, the entire region in the width direction of the sheet leading edge portion abuts on the nip edge line and conforms thereto, thereby effecting skew feed correction. After this, the registration roller is driven to thereby convey the sheet to the image forming unit.
Japanese Patent No. 2893540 discusses a configuration for correcting positional deviation through registration roller shift, which is equipped with a positional deviation detection sensor configured to detect aside end portion in the width direction of the sheet, and a mechanism configured to move the registration roller in a thrust direction (the sheet width direction). And, in the state in which the sheet is pinched by the registration roller, thrust movement is effected until the side end portion in the sheet width direction is detected by the positional deviation detection sensor, whereby positional deviation in the width direction of the sheet is corrected.
However, in such a positional deviation correction device utilizing registration roller shift, the sheet is moved in the width direction while pinched between the upstream roller and the registration roller. Thus, twist is generated in the sheet loop between the two rollers. Due to a reaction force attributable to this loop twist, the sheet may undergo skew feed at the registration roller, or wrinkles or the like may be generated. In particular, in the case of a thick sheet of high stiffness, the loop twist reaction force is large, so that the aggravation of the skew feed is conspicuous.
In view of this, Japanese Patent No. 3191834 discusses a configuration for eliminating the loop twist described above. In this configuration, the upstream roller is supported by a linear bearing member so as to be capable of moving in the thrust direction. Due to this configuration, when the registration roller moves in the thrust direction while pinching the sheet, the upstream roller is also thrust-moved through the sheet, whereby it is possible to eliminate the loop twist. Compression springs are arranged at both end portions of the upstream roller and urge the upstream roller toward the center in the width direction, so that, after the trailing edge portion of the sheet has left the upstream roller, the upstream roller is automatically restored to the former position (the central position in the width direction).
However, the above-described conventional configuration involves the following problem.
The loop twist between the registration roller and the upstream roller is generated not only by the above-mentioned positional deviation correction through the movement in the thrust direction of the registration roller, but also by the skew feed correction through the abutment of the sheet leading edge portion. Specifically, when the sheet undergoing skew feed abuts on the registration roller, the loop is larger at the leading side edge than at the trailing side edge, so that loop twist occurs. As a result, due to a loop twist reaction force, sheet skew feed, wrinkles, etc. may be generated at the registration roller.
It is true that the configuration as discussed in Japanese Patent No. 3191834 is effective as a method for eliminating loop twist. However, the linear bearing member for supporting the upstream roller is rather expensive. Further, the configuration is rather complicated, and the reaction force due to the compression spring urging the upstream roller toward the center in the width direction causes a reduction of the loop twist mitigating effect.
Further, copying machines in recent years have been being more and more reduced in size. Thus, the sheet is nipped not only by the upstream roller but also by a plurality of rollers as the rollers upstream side of the registration roller. Further, in some cases, the sheet trailing edge reaches even the feeding unit. In view of this, it is not realistic to arrange the above-described linear bearing member on every one of the rollers on the upstream side of the registration roller. On the other hand, a sufficient effect cannot be expected when the linear bearing member is only arranged on the upstream roller.
After all, in the configuration in which the skew feed correction through the abutment of the sheet leading edge portion and the positional deviation correction through the movement in the thrust direction of the registration roller are performed at the same time, it is necessary to take into account the superposition of the loop twists due to both corrections. Depending upon the combination of the amounts and directions of these two kinds of loops, the sheet loop twist may be promoted or canceled out. To attain a high-quality image, it is necessary to mitigate the aggravation of the skew feed due to the sheet loop twist.